Various systems and processes are employed to treat sludge and organic waste. For example, sludge and organic waste are sometimes subjected to a thermal hydrolysis process which may occur before or after anaerobic digestion. A thermal hydrolysis process causes cell walls to rupture under conditions of high temperature and high pressure and generally results in highly solubilized sludge which is more easily biodegradable. In particular, thermal hydrolysis employs high temperatures in the range of 130° C. to 180° C. and high pressure, typically in the range of 3-10 bar abs. When used in conjunction with anaerobic digestion, a thermal hydrolysis process decouples long chain polymers and hydrolyzes proteins and generally thermal hydrolysis transforms the sludge or organic waste in ways that makes anaerobic digestion more efficient.
Thermal hydrolysis processes consume huge amounts of energy. Temperature and pressure in the desired ranges for hydrolysis of sludge is typically created by injecting and mixing steam with the sludge in a hydrolysis reactor. Steam injection is a highly energy intensive aspect of sludge hydrolysis and waste energy recovery is a matter of significant concern relative to sludge hydrolysis. In some instances, it is known to partly recover energy by flashing off the steam into the incoming sludge in a batch hydrolysis process or by heating boiler feed water for the production of new steam. These processes are characterized by less than desirable efficiency and very high cost.
Therefore, there is and continues to be a need to improve the energy efficiency of thermal hydrolysis systems and processes.